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Engineering and Technology

2017; 4(5): 56-64

http://www.aascit.org/journal/et

ISSN: 2381-1072 (Print); ISSN: 2381-1080 (Online)

Keywords CFRP Composites,

External Confinement,

FRP Volumetric Ratio,

RC Column

Received: April 18, 2017

Accepted: September 6, 2017

Published: October 13, 2017

Experimental Investigation of Concrete Externally Confined by CFRP Composites

Habib-Abdelhak Mesbah1, Riad Benzaid

2, *

1Laboratory of Civil Engineering and Mechanical Engineering, National Institute of Applied

Sciences of Rennes, University of Rennes 1, France 2Laboratory of Geological Engineering, University of Mohammed Seddik Benyahia, Jijel, Algeria

Email address [email protected] (R. Benzaid) *Corresponding author

Citation Habib-Abdelhak Mesbah, Riad Benzaid. Experimental Investigation of Concrete Externally

Confined by CFRP Composites. Engineering and Technology. Vol. 4, No. 5, 2017, pp. 56-64.

Abstract The present study deals with the analysis of experimental results, in terms of load carrying

capacity and strains, obtained from tests on reinforced concrete (RC) columns,

strengthened with external carbon fiber reinforced polymer (CFRP) sheets. In this work,

we are interested in the effect of the volumetric ratio of the fiber-reinforced polymer (ρFRP)

on the effectiveness of the CFRP external confinement. A total of 36 specimens were

subjected to axial compression. All test specimens were loaded to failure in axial

compression. Compressive stress, axial and hoop strains have been recorded to evaluate the

stress-strain relationship, ultimate strength and ductility of the specimens. Results clearly

demonstrate that for a given confinement level, the increase in cross-sectional dimensions

of the columns would result in a decrease in the volumetric ratio of the fiber-reinforced

polymer (ρFRP), which will result in a decrease in the strength of the externally-confined-

concrete columns. On the other hand, composite wrapping can enhance the structural

performance of RC columns in terms of both maximum strength and ductility.

1. Introduction

The use of fiber reinforced polymer (FRP) confined concrete columns has been proven

in enhancing the strength and the ductility of columns. Over the last two decades, a large

number of experimental and analytical studies have been conducted to understand and

simulate the compressive behavior of FRP confined concrete [1-6]. Such strengthening

technique has proved to be very effective in enhancing their ductility and axial load

capacity. Most of the available experimental data regarding FRP-confined columns have

been generated from tests on small-scale concrete specimens with normal strength [7-14].

So, the validation of these results and their applicability to large-scale RC columns is of

great practical interest. Published work in this field is relatively few [15-17]. More research

investigation is needed on this subject to study the effect of column’s diameter on the

effectiveness of the CFRP external confinement. The present paper deals with the analysis

of experimental results, in terms of load carrying capacity and strains, obtained from tests

on circular reinforced concrete columns, confined with external CFRP composite. The

principal study parameter was the volumetric ratio of the fiber-reinforced polymer (ρFRP).

2. FRP-Confined Concrete in Circular Columns

The confinement action exerted by the FRP on the concrete core is of the passive type,

57 Habib-Abdelhak Mesbah and Riad Benzaid: Experimental Investigation of Concrete

Externally Confined by CFRP Composites

that is, it arises as a result of the lateral expansion of concrete

under axial load.

As the axial stress increases, the corresponding lateral

strain increases and the confining device develops a tensile

hoop stress balanced by a uniform radial pressure which

reacts against the concrete lateral expansion [12, 18]. When

an FRP confined column is subject to axial compression, the

concrete expands laterally and this expansion is restrained by

the FRP. The confining action of the FRP composite for

circular concrete columns is shown in Figure 1. For circular

columns, the concrete is subject to uniform confinement, and

the maximum confining pressure provided by FRP composite

is related to the amount and strength of FRP and the diameter

of the confined concrete core. The maximum value of the

confinement pressure that the FRP can exert is attained when

the circumferential strain in the FRP reaches its ultimate

strain and the fibers rupture leading to brittle failure of the

column. This confining pressure is given by:

2

22 frpfrpfrpfrpfufrpfrp

l

f

d

ft

d

Etf

ρε=== (1)

Where fl is the lateral confining pressure, Efrp is the elastic

modulus of the FRP composite, εfu is the ultimate FRP tensile

strain, ffrp is the ultimate tensile strength of the FRP

composite, tfrp is the total thickness of the FRP, d is the

diameter of the concrete column, and ρfrp is the FRP

volumetric ratio given by the following equation for fully

wrapped circular cross section:

d

t

d

td frpfrp

frp

4

4/2==

ππ

ρ (2)

Figure 1. Confinement action of FRP jacket in circular sections.

3. Experimental Program

3.1. Materials Properties

Concrete mixtures: Three concrete mixtures were used to

achieve the desired range of unconfined concrete strength (26,

50 and 62 MPa), as shown in Table 1. Mixtures were

prepared in the laboratory using a mechanical mixer and

were used to cast the concrete specimens which were

wrapped with CFRP sheets after drying.

Table 1. Concrete mixture proportions.

Mixture no. I II III

Compressive cylinder strength, f’co (MPa) 25.93 49.46 61.81

Cement (kg/m3) 280a 400b 450c

Water (kg/m3) 180 183.86 170

Crushed gravel (kg/m3)

Ø 4/6 122.90 115.70 115.60

Ø 6/12 258.20 243.00 242.80

Ø 12/20 769.50 724.20 723.50

Sand Ø 0/4 (kg/m3) 729.10 686.30 685.60

Sika Viscocrete-Tempo12 (l/ m3), d - 0.85 1.55

Air content (%) 2.3 2.5 2.7

W/C 0.64 0.46 0.37

aPortland cement: CPA CEM II R 32.5 MPa, bPortland cement: CPA CEM I

R 42.5 MPa, cPortland cement: CPA CEM I R 52.5 MPa, dSika Viscocrete-

Tempo 12: High-range water reducing and super-plasticizing admixture.

CFRP composites: The carbon-fiber fabric used in this

study were the SikaWrap-230C/45 product, a unidirectional

wrap. The resin system that was used to bond the carbon

fabrics over the specimens in this work was the epoxy resin

made of two-parts, resin and hardener. The mixing ratio of

the two components by weight was 4:1. SikaWrap-230C/45

was field laminated using Sikadur-330 epoxy to form a

carbon fiber reinforced polymer wrap (CFRP) used to

strengthen the concrete specimens.

3.2. Fabrication of Test Specimens

The experimental program was carried out on circular

columns of 1000 mm height with two different diameters of

160 mm and 200 mm. For all RC specimens the longitudinal

steel ratio was constant for all specimens and equal to 2.25%.

Transverse ties (Ø 8 mm) were spaced every 140 mm. Series

definition and details are given in Table 2.

Table 2. Details of test specimens.

Specimen designation Dimensions (mm) Number of CFRP layers f’co (MPa) Slenderness (L/ Ø)

Col.160-RC26-0L1

Ø160 x 1000

0

26 MPa 6,25

Col.160-RC26-0L2 0

Col.160-RC26-1L1 1

Col.160-RC26-1L2 1

Col.160-RC26-3L1 3

Col.160-RC26-3L2 3

Col.200-RC26-0L1 Ø200 x 1000

0 26 MPa 5

Col.200-RC26-0L2 0

Engineering and Technology 2017; 4(5): 56-64 58

Specimen designation Dimensions (mm) Number of CFRP layers f’co (MPa) Slenderness (L/ Ø)

Col.200-RC26-1L1 1

Col.200-RC26-1L2 1

Col.200-RC26-3L1 3

Col.200-RC26-3L2 3

Col.160-RC50-0L1

Ø160 x 1000

0

50 MPa 6,25

Col.160-RC50-0L2 0

Col.160-RC50-1L1 1

Col.160-RC50-1L2 1

Col.160-RC50-3L1 3

Col.160-RC50-3L2 3

Col.200-RC50-0L1

Ø200 x 1000

0

50 MPa 5

Col.200-RC50-0L2 0

Col.200-RC50-1L1 1

Col.200-RC50-1L2 1

Col.200-RC50-3L1 3

Col.200-RC50-3L2 3

Col.160-RC62-0L1

Ø160 x 1000

0

62 MPa 6,25

Col.160-RC62-0L2 0

Col.160-RC62-1L1 1

Col.160-RC62-1L2 1

Col.160-RC62-3L1 3

Col.160-RC62-3L2 3

Col.160-RC62-0L1

Ø200 x 1000

0

62 MPa 5

Col.160-RC62-0L2 0

Col.160-RC62-1L1 1

Col.160-RC62-1L2 1

Col.160-RC62-3L1 3

Col.160-RC62-3L2 3

The specimen notations are as follows: the first letter

refers to column; the next two letters indicate the type of

concrete (RC for reinforced concrete), followed by the

compressive strength of unconfined concrete f’co (26 MPa, 50

MPa and 62 MPa). The last letters specifies the number of

CFRP layers (0L, 1L and 3L), followed by the number of

specimen.

After 28 days of curing, the CFRP jackets were applied to

the specimens by manual wet lay-up process. The concrete

specimens were cleaned and completely dried before the

resin was applied. The mixed Sikadur-330 epoxy resin was

directly applied onto the substrate at a rate of 0.7 kg/m2. The

fabric was carefully placed into the resin with gloved hands

and any irregularities or air pockets were smoothened out

using a plastic laminating roller. The roller was continuously

used until the resin was reflected on the surface of the fabric,

an indication of fully wetting. After the application of the

first CFRP wrap, a second layer of resin at a rate of 0.5 kg/m2

was applied to allow the impregnation of the second layer of

the CFRP. The following layer is applied in the same way.

Finally, a layer of resin was applied to complete the

operation. The last CFRP layer was wrapped around the

column with an overlap of 1/4 of the perimeter to avoid

sliding or deboning of fibers during tests and to ensure the

development of full composite strength [10, 19]. The

wrapped specimens were left at room temperature for 1 week

for the epoxy to harden adequately before testing. The test

setup is shown in Figure 2.

Figure 2. Test setup.



4. Test Results and Discussion

4.1. Stress-Strain Response

Representative stress-strain curves for each series of tested

CFRP-wrapped columns are reported in Figures 3 and 4. It is

found that for low strength concrete specimens of series 1: 26

MPa (classified as ordinary in some codes), the stress-strain

curves are bilinear with shape softening (Figure 3a, 3b, 4a and

4b). On the other hand, for similar medium or high strength

concrete specimens (Series 2: 50 MPa and Series 3: 62 MPa),

when the strength of the unconfined concrete f'co increases, the

slope of the second branch of the bilinear curve progressively

recovers downward and eventually to a bilinear downward

shape with a substantial reduction in ductility (Figure 3c and 4c).

In the case of stress-strain curves with a bilinear trend. The

first zone is essentially a linear response governed by the

stiffness of the unconfined concrete, which indicates that no

confinement is activated in the CFRP wraps since the lateral

strains in the concrete are very small. Hence the confined and

the unconfined specimens behave in the same manner,

irrespective of the number of layers. After reaching the

maximum load point, the unconfined concrete specimens show

a sudden drop in stiffness and strength. The increase of load

produces large lateral expansions, and consequently the CFRP

wrap reacts accordingly and a confining action is created on

the concrete core. In the second zone, the concrete is fully

cracked and the activated CFRP confinement provides

additional load carrying capacity by keeping the concrete core

intact. The stress-strain curve here increases linearly up to

failure. The stiffness of the specimen in this zone depends on

the modulus of elasticity of the CFRP material and on the level

of confinement (Figure 3a, 3b, 4a and 4b).

(a) RC columns Ø160x1000 mm (series-1: f’co = 26 MPa)

(b) RC columns Ø160x1000 mm (series-2: f’co = 50 MPa)

(c) RC columns Ø160x1000 mm (series-3: f’co = 62 MPa)

Figure 3. Stress-strain curves of FRP confined columns (Ø160x1000 mm).

However, when a concrete column is provided with little

confinement, the bilinear stress-strain behaviour described

above may not exist. Such a situation arises when the

volumetric ratio of FRP composite is very small (Figure 3c

and 4c).

On overall, both ultimate compressive strength and

ultimate strain are variably enhanced depending on the

number of CFRP layers.


(a) RC columns Ø200x1000 mm (series-1: f’co = 26 MPa)

(b) RC columns Ø200x1000 mm (series-2: f’co = 50 MPa)

(c) RC columns Ø200x1000 mm (series-3: f’co = 62 MPa)

Figure 4. Stress-strain curves of FRP confined columns (Ø200x1000 mm).

4.2. Failure Mode

All confined concrete columns failed by fracture of the

composite wrap in a sudden and explosive way preceded by

typical creeping sounds. For all confined specimens,

delamination was not observed at the overlap location of the

jacket, which confirmed the adequate stress transfer over the

splice.

In the category of the (Ø160x1000 mm) reinforced

concrete columns confined with CFRP, the rupture of the

composite envelope was perpendicular to the carbon fibers

followed by circumferential fracture of different widths

which is located generally in the upper or lower half of the

height of the columns (Figure 5). For the concrete columns of

series 2 (50 MPa) confined with 3 CFRP layers, as well as all

series 3 columns (62 MPa), the rupture zone approaches the

middle of the column.



Figure 5. Failure mode of CFRP-confined columns (Ø160x1000 mm).

The failure of the (Ø200x1000 mm) reinforced concrete columns confined with CFRP was generally located in the middle

of the columns for the three series of tested concrete (Figure 6).

Figure 6. Failure mode of CFRP-confined columns (Ø200x1000 mm).

4.3. Effect of Columns Diameter on

FRP-Confinement Effectiveness

In the following, we compare the results obtained for

reinforced concrete columns with circular cross-section

Ø200x1000 mm with respect to columns Ø160x1000 mm for

the three series of concrete tested in the main program of the

present study (series 1: 26 MPa, series 2: 50 MPa and 3: 62

MPa).

4.3.1. Reinforced Concrete Columns

The columns with a diameter of 200 mm recorded an axial

compression strength greater than that of the 160 mm diameter

columns for the three-concrete series (26 MPa, 50 MPa and 62


MPa). Table 3 shows that unconfined columns with a diameter

of 200 mm offer more confinement (lateral stiffness provided

by concrete) than that of columns of 160 mm. This efficiency

of the confinement of the columns Ø200x1000 mm with

respect to that of Ø160x1000 mm decreases with the

compressive strength of the concrete. It was of the order of

69.94% for the concrete of the series 1 (26 MPa) and only of

43.99% for the concrete of the series 3 (62 MPa).

Table 3. Axial compressive strength of RC columns (control specimens).

Concrete

Series

RC columns (f’c200 – f’c160) /

f’c160 (%) Ø 160x1000 mm Ø 200x1000 mm

1 (26 MPa) 25,59 43,49 69,94

2 (50 MPa) 44,99 62,68 39,31

3 (62 MPa) 53,14 76,52 43,99

4.3.2. RC columns Externally Confined with

CFRP

The gains in compressive strength and axial deformation

as a function of the column diameter are shown in Figure 7

for two different confinement levels (1 and 3 carbon fiber-

reinforced polymer layers). Unlike unconfined columns,

those with a diameter of 160 mm confined with 1 or 3 layers

of carbon fiber-reinforced polymer (CFRP) sheets have

achieved gains in compressive strength (f’c) and axial

deformation (εcc) greater than those recorded for the 200 mm

diameter columns as illustrated in Figure 7. However,

confinement with CFRP composite materials reversed the

trend in similar unconfined columns (see Table 3).

Figure 7. Effect of column diameter on the effectiveness of external CFRP confinement.

It should be noted that for a given confinement level, the

increase in cross-sectional dimensions of the specimens would

result in a decrease in the volumetric ratio of the fiber-

reinforced polymer (ρFRP: defined as the ratio of the cross-

sectional area of the composite to that of the concrete), which

will result in a decrease in the strength of the confined concrete.

Figures 8 and 9 show the strength ratio (f'cc/f'co) and the strain

ratio (εcc/εco) as a function of the volumetric ratio of the fiber-

reinforced polymer (ρFRP) for all tested specimens. In general,

increasing ρFRP involves more effective confinement.

However, after confinement with CFRP composite

materials, the 160 mm diameter columns had a ρFRP = 0.0258

and 0.0774 respectively for 1 and 3 layers of CFRP, greater

than those of the columns with a diameter of 200 mm which

had ρFRP = 0.0203 and 0.0609 for 1 and 3 layers of CFRP,

respectively. This explains the confinement efficiency for

columns (Ø160x1000 mm) after confinement with CFRP

composite compared to columns (Ø200x1000 mm) despite

their smaller diameters.

Figures 8 and 9 shows that the variation of the strength

ratio (f'cc/f'co) and the strain ratio (εcc/εco) as a function of the

volumetric ratio ρFRP corresponds in the majority of cases to a

linear function with a coefficient of variation (R2) greater

than 0.90.



Figure 8. Strength and strain ratios depending on the volumetric ratio

(columns with Ø160x1000 mm).

Figure 9. Strength and strain ratios depending on the volumetric ratio

(columns with Ø200x1000 mm).

5. Conclusions

The results of an experimental investigation on the

performance of reinforced concrete columns strengthened

with externally applied uni-directional carbon fiber

reinforced plastic material were presented. The main findings

of this research can be summarized as follows:

a. For a given confinement level, the increase in cross-

sectional dimensions of the columns would result in a

decrease in the volumetric ratio of the fiber-reinforced

polymer (ρFRP), which will result in a decrease in the

strength and ductility of the externally-confined-

concrete columns.

b. The confinement provided by the CFRP improves both

the load-carrying capacity and the ductility of the

column.

c. The failure of all CFRP wrapped specimens occurred in

a sudden and explosive way preceded by typical

creeping sounds.

d. On overall, CFRP strengthened specimens showed a

typical bilinear curve. The first zone is essentially a

linear response governed by the stiffness of the

unconfined concrete. In the second zone, the stiffness of

the specimen depends on the modulus of elasticity of

the CFRP material and on the FRP volumetric ratio.

e. Increasing the amount of CFRP sheets produce an

increase in the compressive strength of the confined

column but with a rate lower compared to that of the

deformation capacity.

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